1. Field of the Invention
The present invention relates to a photoelectric conversion device adapted for use in an image reader, for example a copying machine or a facsimile apparatus, or as an image sensor of a video camera, and more particularly to a photoelectric conversion device for color image reading which eliminates light loss and residual images.
2. Related Background Art
At first there will be explained a conventional solid-state color image sensor, with reference to FIGS. 1A and 1B.
FIG. 1A is a cross-sectional view of an example of a photoelectric converting area of a conventional color MOS solid-state image sensor (for example reported in an article "MOS solid-state image sensors commercialized in color video cameras", S. Nagahara, Optronics (1092) No. 3, p. 15-21). In FIG. 1A there are shown an n-type Si substrate 1; a p-well layer 2; a p.sup.+ layer 3; a field oxide film 4; an n.sup.+ layer 5; a polycrystalline silicon gate 6 for transferring signal charge to a vertical line; an interlayer insulating film 7; a color filter 8, a protective film 9, an n.sup.+ drain 10 for receiving signal charge; a vertical line 11 for signal readout; and a light shielding film 12. The p.sup.+ layer 3 and n.sup.+ layer 5 constitute a photodiode.
As will be apparent from FIG. 1A, within a pixel area A-A', the actual aperture is B-B' so that only a part of the light entering the solid-state image sensor can be converted into the signal charge. The insufficient amount of signal charge has resulted in a limitation in the S/N ratio. This difficulty, becoming more marked as the pixel size grows smaller, has been a major obstacle in realizing a solid-state image sensor which produces a high image quality (namely a high pixel density).
In order to overcome this difficulty, there have been proposed various structures, such as a MOS solid-state image sensor device on which a photoconductive film is laminated (Tsukada T. et al., '79 IDAM, 6.1), and a CCD-type solid-state image sensor device on which a photoconductive film is laminated (Fujimoto S. et al., Color single chip structure for laminate solid-state image sensor, Technical Bulletine of Society of Television TEBS76-2 ED607 p. 7-12, Harada N. CCD-type solid-state image sensor with 2 million pixels for HDTV, laminated by amorphous Si photoelectric conversion film, Nikkei Electronics 1988.2.22).
Among these, a laminate structure on a CCD is shown in FIG. 1B. There are shown a p-Si substrate 21; a p-well layer 22; a channel stop layer 23; a field oxide film 24, wherein the layers 23, 24 serve to separate each pixel; a vertical CCD transfer area; polycrystalline Si layers 26, 27; a Mo electrode 28 for transferring the carriers generated in photoconductive films 29, 30, respectively composed of a ZnSe neubicon film and a ZnCdTe neubicon film, to an n.sup.+ layer 36; a transparent electrode 13; color filters 32, 33 of different spectral transmission characteristics; a protective film 34, and a readout gate area 35. Such structure allows to obtain an aperture rate of 100%.
However the conventional solid-state image sensor devices are still associated with drawbacks such as the limitation in the S/N ratio due to the limited signal charge amount, presence of residual image, and color mixing.
Various experiments of the present inventors have clarified that these drawbacks are ascribable to following three points:
1) the transmittance of the color filter as low as 40 to 70%;
2) presence of flow of signal charge into the adjacent pixel; and
3) a low resetting speed in case of the use of a photoconductive film of a high resistance for improving the point 2).